The transesterification reaction of n-butyl acetate by 1-hexanol catalyzed by Novozym 435 (immobilized Candida antarctica lipase B from Novo Nordisk) suspended in near-critical carbon dioxide, near-critical ethane, and compressed propane at 35 degrees C and 100 bar was studied Separate equilibration of the solvents with mixtures of salt hydrates known to give a certain water activity (a(w)) allowed us to convert water concentration into water activity. In all the solvents, the catalytic activity of Novozym was nearly insensitive to a(w) up to saturation. Reaction rates were higher in propane and ethane than in CO2, possibly due to the direct adverse effect of CO2 already observed with other enzymes. By sieving the enzyme preparation into different size series, it was as possible to verify, the existence of internal diffusion limitations. These were significantly more severe in propane than in the other two solvents which can be explained by the fact that propane behaves like a conventional liquid in this study whereas CO2 and ethane are closer to the critical point. An increase in temperature at constant pressure activated the enzyme in the three solvents. This effect was most pronounced in CO2 and comparable in ethane and propane. in propane, activation by temperature was almost negligible when the whole enzymatic preparation was used which should reflect the existence of more severe mass transfer limitations in this solvent. Experiments at fixed density showed that the activation of Novozym in ethane was solely a temperature effect whereas in CO2, this effect was combined with one mediated by changes in solvation.